Our goal is to describe in further detail the fine structure of the lateral compound eye of the horseshoe crab, Limulus Polyphemus, and to correlate this information with physiological data so abundant in the literature. Toward these ends we are in the process of conducting a comparative study of serial sections from very young animals and adults. Included in these studies is a detailed analysis of the lateral plexus and neuropile and its synaptic organization. Some eyes are being subjected to a specific group of chemicals and drugs in order to examine the effects, if any, of these agents on the morphology and population of synaptic vesicles in the neuropile.